1. Field of the Invention
The present invention relates to optical amplifiers in general and in particular to wideband amplifiers suitable for long haul and ultra long haul optical transmission networks. More particularly, it relates to optical amplifiers for amplifying the full spectrum of the C and L bands combined from 1525 nm to 1610 nm without interruption (middle dead-band).
2. Prior Art of the Invention
Optical amplifiers are used in long haul transmission networks in order to reduce the number of regenerators necessary along transmission lines. The regeneration of optical signals requires their conversion to electronic signals, electronic regeneration, then conversion back to optical signals before retransmission. On the other hand, a number of data channels are transmitted over the optical fiber. As a result at each regeneration station, these channels have to be demultiplexed and go through the regeneration stage one by one. As the number of channels increases, new regeneration modules must be added, if the architecture is scaleable. Efficient optical amplification reduces the need for signal regeneration, which is expensive and a potential bottleneck in optical networking.
Erbium Doped Fiber Amplifiers (EDFA) are the most common type of optical amplifiers used in the long haul WDM (Wavelength Division Multiplexing) and submarine systems. In the first EDFA designs, the wavelength range of 1525 nm to 1565 nm, which is called the xe2x80x9cConventionalxe2x80x9d or C band and is the low-attenuation band of optical fibers, was exploited. However, the need for more bandwidth directed designers"" attention to the longer wavelength range of 1570 to 1610 nm, or the L band. This in turn needed a new type of doped fiber amplifier with a better gain performance in the L band rather than C band. L band fiber amplifiers emerged, which used basically the same technique of the C band amplifiers with specialized doped fibers as the gain medium.
In a typical prior art combined C and L band amplifier, the WDM signal that consists of C band and L band channels is split into C band and L band channels by a C/L band splitter. Each band is then directed to the corresponding fiber amplifier that forms one arm of the combined amplifier. Finally the amplified optical signals for the two paths are combined into the output fiber by a C/L band combiner.
While this type of C and L bands amplifier provides the requisite gain for each of the two bands, when examining the gain across the entire bandwidth, which is from the bottom of the C band to the top of the L band, there is a dip in gain in the gap between the two bands. The middle wavelength range of 1560 to 1570 (also called the dead-band), cannot be used. This is mainly because of the fact that the C/L band splitter filters each band and the middle band of 1560-1570 is in the stop-band of the filter. This wavelength band can accommodate 12 channels for 100 GHz channel spacing or 24 wavelength channels for 50 GHz channel spacing. If each channel carries data at 10 Gbits per second, a total of 120 Gbits per second to 240 Gbits per second data can be transferred on these channels.
The present invention provides an improved combined C and L bands optical amplifiers, which include the optical signals in the middle region (dead-band) between the C band and L bands. In the present amplifiers the C band and L band channels are not separated. The two arms of the optical amplifier have different gain profiles, one for the C band and the other for the L band, and amplify the combined optical signal. Preferably the total combined gain profile is designed to be flat. Moreover, the optical path lengths of the two amplifier arms are made equal preferably by means of a fine tuned delay line.